AC plasma display panels are presently in commercial use as digitally addressable information display devices. The panel itself typically consists of two glass plates with a gas mixture sealed between them. A plurality of X-axis electrodes extend in a mutually parallel array on an interior substrate of one plate, and a plurality of Y-axis electrodes extend in a mutually parallel array on the interior of the other plate. The X-axis electrodes are at a ninety-degree angle to the Y-axis electrodes, thereby forming a plurality of intersections between the X-axis and Y-axis electrodes. A typical commercially available AC plasma panel has 512 X-axis electrodes and 512 Y-axis electrodes, yielding 262,144 intersections.
When a voltage between 180 and 200 volts is applied across an X-axis electrode and a Y-axis electrode, a discharge in the gas occurs at the cell formed by the electrodes, causing a pulse of light to be emitted at this point. Simultaneously, a charge is collected on the cell wall, which results in the cell being an "on" cell. Once such a discharge has been produced and the cell is turned "on", the collected wall charge acts to continue the discharging when a lesser AC sustain voltage is applied between the electrodes. In an "on" cell, the gas will discharge and the cell will emit a pulse of light at each transition of the applied AC sustain waveform. The sustain voltage, however, is insufficient to initiate a discharge at an X-Y intersection. This phenomenon is known as inherent memory, and was originally disclosed by Baker et al., U.S. Pat. No. 3,499,167, and by Bitzer et al., in U.S. Pat. No. 3,959,190. By precisely timing, shaping, and phasing multiple alternating voltage waveforms supplied to X and Y axes electrodes, the generation, sustaining and erasure of light emitting gas discharges at selected locations on the plasma display panel can be controlled.
An AC gas plasma display panel emits light at a sufficiently high intensity to be seen even in areas subject to sunlight. However, since for application of the panel in areas having a low level of ambient light the normal operation brightness of the display panel would be too brilliant, it is desirable to provide means for varying the brightness of the display.
Early attempts to lower the level of the intensity of light emitted from the display panel utilized smoked glass placed over the panel during operation in an area with a low ambient light level. Since plasma display panels may be used in areas subject to both normal sunlight and artificial illumination, this form of brightness control was found to be quite inconvenient, due to the wide variation in levels of illumination experienced.
Later attempts to make a variable intensity plasma-type display utilized several panels having a lower intensity than that of a typical plasma display panel, with the display panels stacked one on top of the other. Such devices require an optical alignment device, which usually is very cumbersome and not wholly effective, as well as being very expensive. This type of device is shown in United Kingdom Pat. No. 1,412,250 to Fujitsu Ltd. Another early type of brightness control used several cells in a plasma display panel as a single unit, selectively turning one through four of them on to vary the intensity of the brightness of the panel. Different applications using this approach are shown in U.S. Pat. No. 3,886,403 to Owaki et al., and U.S. Pat. No. 3,845,243 to Schmersal et al. The disadvantages of such an approach are two-fold: first, additional circuitry is required to selectively drive 25%, 50%, 75% or 100% of the cells in the panel, and this increases both the cost and the complexity of this type of device. The second, and more important, disadvantage of this approach is that the resolution capabilities of the panel are substantially diminished due to the fact that now only one-quarter of the number of picture elements formerly present are available.
The next approaches to variable brightness attempted to provide brightness control by varying parameters of the system. The first approach varied the waveform voltage or current applied to the plasma display panel. U.S. Pat. No. 3,654,388 to Slottow et al describes the use of a variable waveform voltage to control brightness, and U.S. Pat. No. 4,024,529 to Sakai discloses a variable current drive apparatus to vary brightness. The main disadvantage of these approaches is that their complexity makes them difficult to successfully implement on a commercial basis.
The next approach to varying system parameters is best shown in U.S. Pat. No. 3,906,209 to Kurahashi et al., in which the frequency of the sustain voltage is varied. When the frequency is lowered, the number of discharges will also be lowered, and the net amount of light apparent to the human eye is diminished. This approach, however, has a serious disadvantage in that the data rate of the system, in other words, the number of pictures that can be written and erased in a given period of time, is slowed to a point where data cannot be updated often enough to successfully implement a continuously operating system. Kurahashi recognized this, and decreased the sustain frequency of the system only between times when the panel was writing and erasing. However, this approach is unacceptable in a low light environment due to the fact that whenever a write or erase operation is undertaken, light generated will be at full intensity on the screen, possibly momentarily blinding operations of the system.
An offshoot of the frequency variation approach is shown in U.S. Pat. No. 4,149,111, to Coates, Jr. in which the system clock is interrupted in order to decrease the brightness of the display. Unfortunately, this approach completely disrupts the data rate of the system, and is not compatible with a constant data rate brightness control.
The most successful approach to brightness control in the past involved the simple concept of turning on and off elements of the display to control the net brightness of the display over a period of time. An early implementation of this type is disclosed in U.S. Pat. No. 3,570,156 to Easton, in which he writes and erases the display panel over and over. This, of course, presents the obvious disadvantage of not utilizing the inherent memory capabilities of the AC plasma display panel, and requires a large external hardware system. A later approach is shown in U.S. Pat. No. 3,863,023 to Schmersal et al., in which the cells desired to be brightest are turned on first, then the more dim cells are turned on, and finally the dimmest cells are turned on, and then the entire picture is erased simultaneously so that the cells desired to be brightest are turned on for the longest period of time. This, like the Easton device, does not have a true inherent memory display, and would require a large external hardware memory as well as extensive circuitry to control the duration and ordering of cell firings.
A more recent device using the pulse timing concept is disclosed in U.S. Pat. No. 4,006,208 to Fowler et al., in which pulses of different lengths are used in a binary coding scheme to vary the intensity of light emitted from the panel. This device requires extremely complex circuitry, and is not capable of having the display updated as rapidly as would be desired, due to the fact that the use or 64 time slots is required to produce one picture element on the panel.